Fluid separation for water purification or other purposes, is utilized to separate bacteria or other suspended particles from a carrier fluid, or to separate salt or other dissolved substances from the carrier fluid. Such purification can be accomplished by applying the feed fluid under pressure to a membrane that passes substantially fresh water or other permeate. There is usually only a small flux rate, or flow of permeate per unit area of membrane, through the membrane, so that a very large membrane area must be utilized. One technique for applying feed fluid at high pressure while also preventing the build-up of residue on the membrane, is to rotate the apparatus to create high centrifugal forces that help pressurize the feed fluid, help in the radial flow of permeate that has passed through the membrane, and help sweep away concentration polarization and membrane foulant material that could block the membrane. However, it is necessary to produce a very high membrane area per unit volume of a centrifugal device, if such a device is to be made economical enough to gain wide utilization.